This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A112534 #17 Aug 24 2022 09:01:37
%S A112534 1,1,1,3,3,21,7,31,5,480,1526,294,1615
%N A112534 Number of digits in A110778(n).
%e A112534 A110778(4) = 777 hence a(4) = 3.
%t A112534 id[n_]:=IntegerDigits[n]; f[x_,y_]:=FromDigits[Flatten[Append[{x},y]]]; a[x_,y_]:=NestWhile[f[id[#],y]&,f[id[x],y],!PrimeQ[#]&]; le[n_]:=Length[id[n]]; d[x_,y_]:=x-FromDigits[PadRight[id[y],le[x]]]; x=3; t={1}; Do[y=a[x,7]; AppendTo[t,le[d[y,x]]]; x=a[y,3]; AppendTo[t,le[d[x,y]]],{n,5}]; t (* _Jayanta Basu_, May 20 2013 *)
%o A112534 (Python)
%o A112534 from sympy import isprime
%o A112534 from itertools import count, islice
%o A112534 def agen(): # generator of terms
%o A112534 s = ""
%o A112534 while True:
%o A112534 for d in "37":
%o A112534 for k in count(1):
%o A112534 if isprime(int(s+d*k)): break
%o A112534 yield k
%o A112534 s += d*k
%o A112534 print(list(islice(agen(), 10))) # _Michael S. Branicky_, Aug 23 2022
%Y A112534 Cf. A110774, A110775, A110776, A110777, A110778, A110780, A110781, A110782, A110783, A110788, A110789.
%K A112534 base,nonn
%O A112534 1,4
%A A112534 _Joshua Zucker_, Jan 12 2006
%E A112534 Typo in example fixed by _Sean A. Irvine_, Mar 21 2010
%E A112534 a(11)-a(12) from _Sean A. Irvine_, Mar 22 2010
%E A112534 a(13) from _Michael S. Branicky_, Aug 23 2022